Electrolyte for and method of polishing metal surfaces anodically



Patented MaylZ, 1942 UNITED STATES PATENT OFFICE 2,282,351

nmc'raonm roa AND mr'rnon or r01.-

rsnmo METAL summons anomomr Charles 1.. Faust, Columbus, out, assignor to Battelle Memorial Institute,

corporation of Ohio Columbus, Ohio, a

No Drawing. Application mm 5, 1940, Serial No. 359.930

. I 8 Claims. (01. 204-140). This invention relates to an electrolyte for and to a method .of finishing metal surfaces anodically. More particularly the invention relates to the anodic treatment of metals in an aqueous electrolyte containing arsenic acid or the equiva lent arsenate radicals having a total acid concentration, furnished either by arsenic acid itself, or by an admixture of arsenic acid with sulfuric, phosphoric, chromic or other suitable acid, of from 50% to 95% by weight. By the" selecsenic acid and one or more of the other acids tion of an electrolyte composition containing arnamed, within the limits of total acid concentration specified, it is possible to impart to the surface of a'metal, when treated as an anode therein, a lustrous, mirror-like polish. a

This constitutes a continuation in-part of my application of the same title, Serial No. 290,787, filed August 18, 1939.

It has heretofore been proposed to use various compositions of electrolyte for the anodic treatacteristics of mechanically polished metal surfaces. 1

. It is a further important object of this invention to provide a method for the anodic polishing of metals by making the metal to be polished the anode in an electrolyte of relatively high acid concentration containing arsenic acid, or equivalent arsenate radicals, and one or more mineral acids.

Other and further important objects of this invention will become apparent from the 101-" lowing description and appended claims.

A method to which this invention pertains involves making the metals to be treated an anode in an electrolytic bath of suitable composition and passing a current therethrough of sufficient density and for a sufiicient length of time to produce the desired type of surface finish, which,

more specifically may be a highly lustrous or mirror-like polish.

The electrolyte that I have found to possess the most general applicability comprises an aqueous solution of arsenic acid, either by itself or in ad-- mixture with sulfuric acid, phosphoric acid, or

'chromic acid, or two or more of such acids. The term arsenic acid is intended to include any of the various forms of arsenic acid, such as ortho,

meta and pyro, that may be used as such in 'making up the aqueous electrolyte or that may be a dense, adherent layer of aluminum oxide as a protection therefor against corrosion and wear.

I have now found that an electrolyte containing arsenic acid, or an equivalent arsenate radical, in an aqueous acid solutiomof sufficient total acid concentration can be satisfactorily employed in the anodic treatment of various metals and alloys to impart to the surfaces thereof the desired type of finish. In the preferred embodiment of my invention, which will be described more specifically hereinafter, such an electrolyte is employed for imparting a lustrous, mirror-like surface, but its use is not limited to that purpose alone.

It is therefore an important object of my invention to provide an electrolytic solution containing arsenic acid, or arsenate radicals, and having a relatively high total acid concentration for use in the anodic treatment of metals generally to impart thereto a desirable surface finish.

It is a further important object of myv invention to provide an electrolyte for and a method of anodically treating metals and alloys to produce thereon surfaces that are highly lustrous and formed in the electrolyte asthe result of the action of the strong mineral acid or acids, upon the particular arsenate compound added. For satisfactory results, the concentration of arsenic acid (HaAsOi), should be between 5 and 93% by weight of the solution, and if used alone, the

arsenic acid concentration should be at least 50% by weight of the solution. It is generally preferable to include one or more of the other mineral acids in the electrolyte, and, in that case, thecombined acid concentration should be between 50 and 95% by weight of the solution. The mineral acids preferred for "admixture with the arsenic acids are sulfuric acid within the range of from 0 to 85%, phosphoric acid withinfthe range of 0 and 85%, and chromic anhydride .(CrOa) within the range of from 0 to saturation.

If it is desired to obtain the highest degree of polish of surfaces of the metals or alloys undergoing treatment, the minimum combined acid concentration should be not less than 60% by weight. Aside from such metallic salts as may be present inthe solution due to anodic dissolution of the metal undergoing treatment, the balance of the electrolytic solution will be largely water. There free from the scratches and piled" layers charwill thus be a minimum of 5% and a maximum of 50% of'water 'in the electrolyte of my inven- This bath produced a mirror finish on stain-- tion. less steel at current densities of 250 amps/sq. ft. While it is possible to produce desirable surand a temperature of around 50 t 709 c when face efiect with mixtures of arsenic and other the m tal was treated anodicaliy for 40 minutes.

acids containing as h as 50% water, I Prefer 5 A mirror finish was also produced on copper at a to keepthe water content of the electrolyte relacurrent density of 500 ampsilsqaft. operated for tiv'ely low, since I have found, in general, the bath 23 minutes. 7

containing the lower proporgions of water may Example! be operated at lower current ensities, and there- The rol'lowm a g bath was found to be effective fore at a lower cost for electrical current. a. v 10 polishing brass when the brass was treated It Wm be understood or course that during i anodically for thirty minutes at a current density 7 the operation of an electrolyte such as described, there will be a gradual building up of the content' of amPST/Sq of metal salts formed by dissolution of the metal Arsenic arm I Per 9 undergoing anodic treatment. There also may orthophosphonc acid I 55 r be some change in the water content due to loss chrome and 3 'of water by evaporation or as a result of elecwater '7 27 trolytic decomposition or due to gain of water by condensation or collection of moisture from i I I P 5 I the air. The composition of the electroly e 2 A desirable finish can be produced an alushould, however, be kept within the foregoing min b 6 tin rammjn m di 1 limits as to total acid concentration and water a g? i i f fig i cal y m content if satisfactory operating conditions are I Per cent to be maintained I I I Arsenic acid I The following will serve to illustrate preferred 25 sulfuric acid 15 compositions of my electrolyte: rthophospilcilozicorggd i3 v Chromic ac Example 1 water 15 -F01 use in the electropolishing of stainless steel, rustless iron, nickel and nickel alloy, I have 30 is g g 3 52 gg gg gfg f found that any one fi f" Q is In general, anode current densities varying rely satisfactory: I from 50 to 1000 amps./sq. ft. may be employed,

- a depending upon the type of surface treatment Bath 3 that is desired, the typeof metal or alloy undergoing treatment and the composition of the elec- 1 2 trolyte. Higher current densities than indicated e a by the foregoing range may also be employed, but

, sulphuric, gg g; "W5 in that case the higher current cost offsets the Arsenic acid HiAso. 47 69 '11 40 saving in time. The length of time to efiect the 17 6 13 desired results depends upon the magnitude of the current density employed, and to some extent h the or oin compositions of el tr yt upon the particular metal or alloy undergoing excellent P 8 8 have e o i-awed 0n treatment and the character of the metal sur- 2 2 Chromium-nickel stainless Steels, Plain faces initially. Rough surfaces, 'of course, re-

carbon ste ls, opp b ass, ic el. d nickel quire a longer time to polish than relatively alloys, such as Monel metal, and Chromel, opersmooth ones.

M1118 thebath at atemperature' of As illustrative of the length of time required a d e p oyin a ode c e density of f to effect a high degree of electropolish, from 5 to to 100 amperes per square foot and upwards. 5o 90 minutes is usually found suflicient. The tem- Examp'le 2 perature at which the anodic treatment is carried out, while preferably between 50 and 75 C., may .The following bath was effective in the electrobe varied from around mom temperature to Polishing of copper where metal was made peratures only slightly under the boiling point of an anode with a current density of 385 amperes water;

per square foot and a temperature of C.' for I From the f g i description of my invem fifteen n e tion, it is apparent that I have provided a novel P cent composition of electrolyte for use in the anodic Arsenic a 84 I treatment of metals. Chromic acid (C1 2 1 While an aqueous solution of arsenic acid alone Water 6 is satisfactory for polishing some metals and m .same was also efiective in I alloys, as for instance stainless Steel, mm at a f t density of 340 amperes per preferable to use another acid with it, such as square foot at a temperature of 63 C. for eight enhydride, sulfuIrie acid, Or O-DhQS- minutes. 'Alloys of zinc and copper, stainless 65 Phone acid. Where a mixture of only two acids 1 steel'and nickel alloys could alsobe polished i is used, such as arsenic-chromic, arsenic-sulfuric,

t t under comparable conditions I or arsenic-phosphoric, the limits for ,the arsenic Ezam 3 acid areas above stated, viz. from 5 to 93%; the

p e limits for chromic anhydride from 0.1% to satue fOIIOWing bath was'efieetive i the polishration; the" limits for sulfuric acid from 5 to ing of 18-8 type of stainless steel andcopper: 5%; and t fo ortho pho h ri acid I s I p cent from 5 to The combined acid concentra-v Arsenic acid 15 tion in any case should be between 50 and Orthophosphoric acid 60 It will, of course, be understood that various Water 25 details of the process may be varied through awide range'without departing from the principles of this invention and it is, therefore, not the purpou to limit the patent granted hereon otherwisethannecessitatedbythescopeoftheappended claims. I

' I claim as my invention:

1. An electrolytefor use in the anodic polishing of metals, comprising an aqueous solution of about 84% arsenic acid, chromic anhy-' dride and 6% water by weight.

2. An electrolyte for use in the anodic polishing of metals, comprising an aqueous solution of about arsenic acid, 3% chromic acid, 55% orthophosphoric acid, and 27% water by weight.

3. An electrolyte for use inthe anodic polishing of metals, comprising an aqueous solution of .about arsenic acid, 15% sulfuric acid, 35%

orthophosphoric acid, 10% chromic anhydride and 15% water. by weight.

'4. The method of electropolishing metals, which comprises making a metal the anode in an electrolyte comprising about 84% of arsenic acid,

10% of chromic acid and 6% of water,'by weight.

and passing a current therethrough of suillcient density and for a suillcient length of time to effect a polish onsaid metal surface.

5. The method of electropolishing metals,

which comprises making a metaithe anode in an electrolyte comprising about 15% of arsenic acid, 3% of chromic acid, 55% of orthophosphorlc acid and 21% of watenbyweight, and passing a current therethrouah of-suilicient density and for a milicientlengthoftimetoeifectapoiishonsaidmetal surface.

6. The 'method of electropolishing metals, which comprises making a metal the anode in an electrolyte comprising about 25% of arsenic acid, 15% of sulfuric acid, of orthophosphoric acid, 10% of chromic anhydride and 15% of water, by weight, and passing a current therethrough of suilicient density and for a sufllcient length of time to effect a polish on said metal surface. v

7. An electrolyte for use in the anodic polishing of metals comprising an aqueous solution of from 5 to 93% of arsenic acid, from 0 to 85% of sulfuric acid and a significant amount of an acid selected from the group consisting of phosphoric acid and chromic acid and mixtures thereof, said solution having a combined acid concentration of at least by weight, the balance beingv larsely water.

8. The method of anodically p lishing metals, which comprises making a metal the anode in an aqueous acid electrolyte comprising from 5 to 03% arsenic acid, from 0 to of sulfuric acid and a. significant amount of an acid selected from-the group consisting of ph horic acid and chromic acid and mixtures thereof, said electrolyte having a combined acid concentration of at least 50% by weight, the balance being largely water and passing an electric current therethrough of a sumcient density and for a suiilcient Period of time to eflect'the desired polish.

CHARIBB L. FAUBT. 

